In recent years, electric double layer capacitors capable of charging and discharging with a large current have been attracting attention as a type of electric power storage device for applications requiring frequent charge/discharge cycles, for example, auxiliary power supplies for electric vehicles, solar cells, wind power generation, etc. There is therefore a need for an electric double layer capacitor that has high energy density, is capable of fast charging and discharging, and has excellent durability.
An electric double layer capacitor comprises a pair of polarizable electrodes as a positive electrode and a negative electrode disposed opposite each other with a separator interposed therebetween. Each polarizable electrode is impregnated with an aqueous or non-aqueous electrolytic solution, and is combined with a current collector.
A carbon material having a graphite-like microcrystalline carbon is known for use as a polarizable electrode material for electric double layer capacitors (Japanese Unexamined Patent Publication No. H11-317333). This carbon material is prepared by controlling the activation process of the raw material so that the distance between the layers of the crystallites of the graphite-like microcrystalline carbon lies within a range of 0.350 to 0.385 nm. A microcrystalline carbon having this specific interlayer distance exhibits the property that when the usual expected operating voltage (rated voltage) is applied while holding the carbon in contact with an electrolytic solution, only a small capacitance can be obtained because of its small specific surface area, but once a voltage higher than the expected operating voltage is applied, electrolyte activation occurs with electrolyte ions inserted between the layers, thus producing a high capacitance. Once the ions are inserted, the carbon material maintains high-capacitance even when it is repeatedly used with the expected operating voltage. Compared with activated carbon commonly used as a carbon material for electric double layer capacitors, the above carbon material has a high withstand voltage and permits energy density to be increased significantly, and therefore has been attracting attention as a carbon material that can replace activated carbon.
When performing initial charging to complete the electric double layer capacitor whose polarizable electrodes are formed from the carbon material having graphite-like microcrystalline carbon, it is known to use a voltage higher than the expected operating voltage in order to forcefully insert electrolyte ions between the layers of the microcrystalline carbon (Japanese Unexamined Patent Publication No. 2000-077273). It is also known that, during the initial charging of the electric double layer capacitor, a pressure that resists the expansion of the polarizable electrodes due to the insertion of the electrolyte ions is applied to the polarizable electrodes in order to suppress the expansion of the polarizable electrodes (Japanese Unexamined Patent Publication Nos. 2000-068164 and 2000-068165). Furthermore, it is known that, when performing such initial charging, the electric double layer capacitor is charged for a time longer than the rated charge time of the electric double layer capacitor (Japanese Unexamined Patent Publication No. 2000-100668). Any of these prior art techniques is intended to improve the method of initial charging in order to increase the capacitance of the electric double layer capacitor. However, no prior art technique is known that focuses on the processing of the polarizable electrodes before charging or the temperature during the charging.